1. Field of the Invention
The present invention falls within the technical field relating to a periphery monitoring system which uses a Doppler sensor for monitoring movements of a mobile object around the installation location of the Doppler sensor to estimate, for example, intrusion of the mobile object into a compartment of a vehicle.
2. Description of the Prior Art
Conventionally, a system for detecting intrusion into a compartment of a vehicle wherein when a door of the vehicle is opened the system determines it as an abnormality to give an alarm, for example, has been well known. There has also been a system for detecting intrusion of a human into a vehicle compartment by utilizing a Doppler shift which is caused when the human intrudes into the vehicle compartment as disclosed in, for example, Japanese Laid-Open Patent Publication No. 2004-181982.
However, the above conventional intrusion detection systems can detect intrusion only after a human has intruded into a vehicle compartment, and therefore, it is too late to give an alarm in many cases.
In view of such, utilization of the Doppler shift in Japanese Laid-Open Patent Publication No. 2004-181982 can be further developed into a concept that intrusion of a mobile object, such as a human, or the like, is estimated in advance using a Doppler sensor and, when the possibility of intrusion is expected, some reaction, such as an alarm, or the like, is made. This Doppler sensor emits a transmission wave, such as a micro wave, or the like, and receives a reflection of the transmission wave by an object to output as a signal the shift in frequency between the transmission wave and the reflection wave (which varies according to the moving velocity of the object). The signal output from the Doppler sensor is subjected to an FFT analysis, through which a predetermined frequency range is divided into a predetermined number of frequency bands, each having a predetermined bandwidth. The frequency level is obtained for each frequency band, and movements of a mobile object are grasped based on a variation in the frequency levels of the frequency bands (especially, the total sum of the frequency levels of all of the frequency bands).
However, the Doppler sensor is susceptible to influences of mobile phone waves, radio broadcasting waves, and the like, and has a high possibility of falsely detecting an abnormality because of the influences. For example, when a mobile phone is turned on in the vicinity of a vehicle, the noise level including the wave from the mobile phone increases at the moment, and accordingly, the total sum of the frequency levels of all of the frequency bands increases. This phenomenon is generally the same as that caused when a human approaches to a vehicle and is therefore difficult to distinguish from movement of a human.